Abstract: The present invention is directed to a method for preparing an expression vector encoding a tailored recombinase, wherein said tailored recombinase recombines asymmetric target sites within the LTR of proviral DNA of a retrovirus inserted into the genome of a host cell and is useful as means for excising the provirus from the genome of the host cell. The present invention further relates to an in vitro-method of optimising the treatment of a retroviral infection of a subject and to the use of tailored recombinases for the preparation of pharmaceutical compositions for reducing the viral load in a subjected infected by a retrovirus.